Conventional resins used in a pultrusion process for making a fiber reinforced resin composite are thermoset resins, such as unsaturated polyester resin and epoxy resins. Phenolic resin which is known to have excellent mechanical properties and electrical properties, especially at elevated temperatures, has been used to produce fiber reinforced products by prepreging, compounding, reaction injection molding or pultrusion processes, but phenolic resin used as a binder in the fabrication of a fiber reinforced composite product so far is still not so popular compared to the others due to its brittle nature which causes its poor coupling with fiber, its slow curing rate, and due to the side product (water) which may be generated causing void defects. Therefore there is a need in the composite industry to improve the quality and properties of a pultruded fiber reinforced phenolic based resin composite.
Several methods have been developed to toughen phenolic resin. For examples, U.S. Pat. No. 2,267,390 uses China wood oil (tung oil), Japanese Patent No. 29-7595 uses rosin, and U.S. Pat. No 2,675,335 uses alkyl phenol for toughening phenolic resin. However, these methods all require the modifying agents to react with phenolic resin for a period of time in order to obtain the desired toughening effect.
At present meta-hydroxyl phenol catalyst is used to shorten the gel time of phenolic resin so that it can be used in manufacturing a fiber reinforced composite product; however, the addition of meta-hydroxyl phenol catalyst also decreases the storage stability of phenolic resin and pot life of phenolic resin.
It is known that the coupling between phenolic resin and the surface of a common fiber is poor, and filaments having a special surface treatment are necessary for the fabrication of a fiber reinforced phenolic resin, e.g. filaments having a special surface treatment by isocyanate based coupling agent are available from the fiber glass manufacturers, e.g. Clark-Schwebel Fiberglass Corp., U.S., etc. These special treated filaments result in more stocks and production costs, and an additional operation procedure of changing different filaments.
To our knowledge there is no method in the art which can toughen phenolic resin, increase the curing rate of phenolic resin without adversely affecting the storage stability thereof, and enhance tile coupling between phenolic resin and fiber at the same time.
The present inventors are the first ones who use phenolic resin modified by blocked polyurethane to prepare fiber/resin composites. The blocked polyurethane can enhance the toughness, the mechanical properties of the fiber/resin composite, and the curing rate of the modified phenolic resin without adversely affecting the storage stability thereof. Moreover, the filaments having a special surface treatment are no longer needed in the preparation of a fiber reinforced phenolic resin composite by using the modified phenolic resin because a good coupling between the modified phenolic resin and regular filaments can be obtained.